1. Field of the Invention
The present invention relates to a halftone picture processing apparatus for automatically detecting a halftone region in an image signal obtained upon scanning an original such as a halftone picture containing an image area the density of which is represented by dot size.
2. Brief Description of the Prior Arts
Most general gray scale or color images provide printouts utilizing halftone pictures. In general, halftone pictures are used in the field of printing to express the density of an original. By using different sized dots, a continuous-tone picture can be represented by a halftone picture. Halftone pictures are thus used for most printouts. Ink dot patterns vary from fine to rough. When a continuous-tone picture is scanned by a facsimile system, the period of gradation level changes is large enough as compared with the sampling period so that the change in gradation between adjacent pixels is small. However, a halftone picture is constituted by an aggregate of small black dots, the density of which is substantially the same as the sampling period. For this reason, the gradation abruptly changes from pixel to pixel. When a halftone picture is transmitted by a facsimile system or the like, or is encoded and stored as an image file in a memory, existing coding schemes assume that gradation changes are the same as in continuous-tone pictures As a result, the conventional coding schemes are not suitable for halftone pictures and greatly impair coding efficiency.
When a gray scale picture is accessed at a binary terminal, a halftone picture is represented by a white and black representation (so-called dithered picture). In this case, moire noise degrades image quality. Furthermore, an original may include halftone, continuous-tone and document regions mixed together. In order to prevent the above problem, halftone regions in the original must be detected, and the same processing as for continuous-tone pictures must be performed for the halftone picture after variations caused by screen pattern are eliminated from the image signal. As a result, halftone detection is absolutely necessary. A halftone screen frequency is 65 to 200 lines/inch. If a scanner frequency of facsimile is given as 8 dots/mm, the ratio of screen to halftone frequency varies from 4.4 to 1.4. An effective halftone detection method is thus required for different halftone screen frequencies.
The following three conventional halftone detection schemes are currently available:
(1) Half Screen Frequency Detection Scheme
An image is transformed to coordinates in an orthogonal coordinate system, and the coordinates are plotted along the spatial frequency axis to provide different features in accordance with different images. When the halftone picture is converted and plotted in the orthogonal coordinate system, the halftone region can be detected since the halftone picture has a component of halftone screen frequency in the high-frequency region. This conventional scheme, however, requires many reference pixels to obtain sufficient data for detection.
(2) Scheme for Comparing Input Signal with Reference Halftone Screen Pattern
Signals representing characteristic screen patterns are stored in a memory or the like. An input signal is compared with the stored screen patterns. A halftone region is detected in a portion where the input signals frequently coincide with the screen patterns. This conventional scheme requires a large memory capacity, and the screen patterns must be known in advance, thus impairing versatility.
(3) Level Distribution Type Detection Scheme
Halftone pictures are obtained by spatially modulating a continuous-tone picture with black dots. Thus, the occurrence frequency of pixels with high or low densities is very high, and the occurrence frequency of pixels with intermediate densities is very low. According to this conventional scheme, level distribution characteristics are utilized to detect halftone regions. This conventional scheme, however, requires many pixels to obtain sufficient data for detection.
According to another conventional scheme, image signals which do not contain halftone regions, such as character portions and photographs, are divided into an abrupt level change portion, including edges and characters, and a moderate level change portion, excluding edges and characters. Averaging of the image signal is then performed. In this manner, redundancy is eliminated without impairing precision. However, due to quick changes in signal levels of the halftone regions described above, this scheme cannot detect edges or sufficiently eliminate redundancy in averaging without impairing precision for an image signal including halftone regions.